The present invention relates to energy conversion systems, and in particular, to a degaser for removing noncondensable gases from an elevated temperature fluid preparatory to introducing the fluid into a heat exchanger.
In the above energy conversion system the heat exchanger can be a direct-contact, single or binary-fluid heat exchanger for transfering energy from the elevated temperature fluid, as for example brine from a geothermal well, to an immiscible working fluid, as for example, isobutane. The isobutane is vaporized in the heat exchanger and can be used to drive a turbine. The energy conversion system further include a condenser for condensing the working fluid preparatory to the fluid being reintroduced into the heat exchangers.
Noncondensable gases including carbon dioxide, can be dissolved in and carried with the geothermal brine. These gases, if not properly removed from the system, can degrade the performance of the heat exchanger in the following several ways.
First, total pressure of the gases leaving the heat exchanger and entering the turbine equals the summation of the partial pressures of vaporized isobutane, of any escaping brine vapor, and of the noncondensable gases. As the noncondensable gases in the geothermal brine usually vary in amount and composition, a turbine designed for an average amount and composition will usually be operating on a nonaverage amount and composition due to the fact that an average amount and composition may not exist, but merely be a mathematical midpoint. Accordingly, the turbine will usually be operating inefficiently. Further, due to the presence of the noncondensable gases, the turbine may be operating in a corrosive environment.
Second, the total pressure of gases in an unvented condenser, which condenses the isobutane or like hydrocarbon after the isobutane exits the turbine, builds up to an equilibrium value that depends on the rate at which the gases, including the noncondensables, entering the system can be removed by redissolving the same in the cool exiting brine. This buildup of gases at the condenser interferes with heat transfer and requires a much larger condenser. Venting the condenser, however, can be very costly in terms of equipment and the losses of the working fluid.
Third, the buildup of back pressure at the condenser badly degrades the performance of the turbine. In isentropic expansion, the work output of the turbine is roughly proportionaly to the logarithm of the pressure ratio. The back pressure sharply reduces this ratio with a consequent loss of power.
Fourth, the back pressure of the noncondensables interferes with a flashing process in the direct-contact, binary-fluid heat exchanger by which the geothermal brine actively transfers its heat to vaporize the isobutane. Consequently, for the same brine temperatures and for equivalent transfers of heat, higher total pressures and larger equipment are required.
Fifth, the presence of the noncondensables in the geothermal brine entering the heat exchanger encourages froth formation in the heat exchanger with a resultant lowering in thermal conductivity.
Sixth, as indicated above some of the gases are corrosive, and if allowed to enter the isobutane loop would require costly construction to prevent damage to the equipment.
Seventh, recovery of dissolved isobutane from the spent geothermal brine by boiling in vacuo and condensing is feasible provided that the geotheral brine is essentially free of noncondensable gases. Without this recovery of isobutane, however, the direct contact cycle is neither economically viable nor environmentally acceptable.
The prior art has attempted to deal with these problems in several ways. One approach is the venting off of some of the noncondensable gases at a well head separator or at a small vent tank directly ahead of the direct-contact, binary-fluid heat exchanger. These devices have proven hard to control and fail to remove all of the noncondensable gases and also result in a loss of steam from the geothermal brine.
A prior art device produced by the Ben Holt Company has been proposed as a means for saving the steam (see Anker v. Sims 1976--Geothermal Direct Contact Heat Exchanger--pp 48-62, Final Report, the Ben Holt Co., SAN/1116-1 for Energy Research & Development Adm., DGE June 10, 1976). In the Ben Holt system, part of the steam is used to preheat and boil a small side stream of the isobutane by means of a special heat exchanger necessarily consisting of an isobutane boiler section and a counter-current isobutane preheater and noncondensable gas subcooler section. The isobutane vapors resulting are directed to the turbine. The rest of the steam is used in another heat exchanger to preheat water for a plate-type scrubber for washing salty mist out of the isobutane vapor stream ahead of the turbine. Thus three separate pieces of equipment in addition to a vent tank or brine flash separator with much interacting piping, values, and controls are required.
In addition, the Ben Holt system is inefficient due to the pressure drop and temperature loss associated therewith. As for example, in the Ben Holt system a back pressure regulator or pressure reducing valve must be placed between the vent tank and the special heat exchanger. In operation, this regulator causes the steam to condense at a lower temperature than is required to boil the isobutane. Accordingly, the isobutane is not boiled and there is an associated reduction in system efficiency.